Identification and characterization of a rat decidual insulin-like growth factor-binding protein complementary DNA

Atlas of tissue- and developmental stage specific gene expression for the bovine insulin-like growth factor (IGF) system

The insulin-like growth factor (IGF) axis is fundamental for mammalian growth and development. However, no comprehensive reference data on gene expression across tissues and pre- and postnatal developmental stages are available for any given species. Here we provide systematic promoter- and splice variant specific information on expression of IGF system components in embryonic (Day 48), fetal (Day 153), term (Day 277, placenta) and juvenile (Day 365–396) tissues of domestic cow, a major agricultural species and biomedical model. Analysis of spatiotemporal changes in expression of IGF1, IGF2, IGF1R, IGF2R, IGFBP1-8 and IR genes, as well as lncRNAs H19 and AIRN, by qPCR, indicated an overall increase in expression from embryo to fetal stage, and decrease in expression from fetal to juvenile stage. The stronger decrease in expression of lncRNAs (average ―16-fold) and ligands (average ―12.1-fold) compared to receptors (average ―5.7-fold) and binding proteins (average ―4.3-fold) is consistent with known functions of IGF peptides and supports important roles of lncRNAs in prenatal development. Pronounced overall reduction in postnatal expression of IGF system components in lung (―12.9-fold) and kidney (―13.2-fold) are signatures of major changes in organ function while more similar hepatic expression levels (―2.2-fold) are evidence of the endocrine rather than autocrine/paracrine role of IGFs in postnatal growth regulation. Despite its rapid growth, placenta displayed a more stable expression pattern than other organs during prenatal development. Quantitative analyses of contributions of promoters P0-P4 to global IGF2 transcript in fetal tissues revealed that P4 accounted for the bulk of transcript in all tissues but skeletal muscle. Demonstration of IGF2 expression in fetal muscle and postnatal liver from a promoter orthologous to mouse and human promoter P0 provides further evidence for an evolutionary and developmental shift from placenta-specific P0-expression in rodents and suggests that some aspects of bovine IGF expression may be closer to human than mouse.

Insulin-like growth factor (IGF) binding protein-4 is both a positive and negative regulator of IGF activity in vivo

IGFs are required for normal prenatal and postnatal growth. Although actions of IGFs can be modulated by a family of IGF-binding proteins (IGFBPs) in vitro, these studies have identified a complicated pattern of stimulatory and inhibitory IGFBP effects, so that understanding relevant aspects of IGFBP action in vivo has been limited. Here we have produced a null mutation of one specific IGFBP, IGFBP-4, which is coexpressed with IGF-II early in development. Surprisingly, mutation of IGFBP-4, believed from in vitro studies to be exclusively inhibitory, leads to a prenatal growth deficit that is apparent from the time that the IGF-II growth deficit first arises, which strongly suggests that IGFBP-4 is required for optimal IGF-II-promoted growth during fetal development. Mice encoding a mutant IGFBP-4 protease (pregnancy-associated plasma protein-A), which facilitates IGF-II release from an inactive IGF-II/IGFBP-4 complex in vitro, are even smaller than IGFBP-4 mutant mice. However, the more modest IGFBP-4 growth deficit is completely restored in double IGFBP-4/pregnancy-associated plasma protein-A-deficient mice. Taken together these results indicate not only that IGFBP-4 functions as a local reservoir to optimize IGF-II actions needed for normal embryogenesis, but also establish that IGFBP-4 proteolysis is required to activate most, if not all, IGF-II mediated growth-promoting activity.

Expression of insulin-like growth factor system genes during the early postnatal neurogenesis in the mouse hippocampus

Insulin-like growth factor-1 (IGF-1) is essential to hippocampal neurogenesis and the neuronal response to hypoxia/ischemia injury. IGF (IGF-1 and -2) signaling is mediated primarily by the type 1 IGF receptor (IGF-1R) and modulated by six high-affinity binding proteins (IGFBP) and the type 2 IGF receptor (IGF-2R), collectively termed IGF system proteins. Defining the precise cells that express each is essential to understanding their roles. With the exception of IGFBP-1, we found that mouse hippocampus expresses mRNA for each of these proteins during the first 2 weeks of postnatal life. Compared to postnatal day 14 (P14), mRNA abundance at P5 was higher for IGF-1, IGFBP-2, -3, and -5 (by 71%, 108%, 100%, and 98%, respectively), lower for IGF-2, IGF-2R, and IGFBP-6 (by 65%, 78%, and 44%, respectively), and unchanged for IGF-1R and IGFBP-4. Using laser capture microdissection (LCM), we found that granule neurons and pyramidal neurons exhibited identical patterns of expression of IGF-1, IGF-1R, IGF-2R, IGFBP-2, and -4, but did not express other IGF system genes. We then compared IGF system expression in mature granule neurons and their progenitors. Progenitors exhibited higher mRNA levels of IGF-1 and IGF-1R (by 130% and 86%, respectively), lower levels of IGF-2R (by 72%), and similar levels of IGFBP-4. Our data support a role for IGF in hippocampal neurogenesis and provide evidence that IGF actions are regulated within a defined in vivo milieu.

Delayed mammary gland involution in mice with mutation of the insulin-like growth factor binding protein 5 gene

IGFs (IGF-I and IGF-II) are essential for development, and their bioactivities are tightly regulated by six related IGF-binding proteins (IGFBPs). IGFBP-5 is the most highly conserved binding protein and is expressed in several key developmental lineages as well as in multiple adult tissues including the mammary gland. To explore IGFBP-5 actions in vivo, we produced IGFBP-5 knockout (KO) mice. Whole-body growth, selected organ weights, and body composition were essentially normal in IGFBP-5 KO mice, presumably because of substantial compensation by remaining IGFBP family members. The IGFBP-5 KO mice also exhibited normal mammary gland development and were capable of nursing their pups. We then directly evaluated the proposed role of IGFBP-5 in apoptosis and remodeling of mammary gland during involution. We found that the process of involution after forced weaning was delayed in IGFBP-5 KO mice, with both the appearance of apoptotic cells and the reappearance of adipocytes retarded in mutant mice, compared with controls. We also determined the effects of IGFBP-5 deletion on mammary gland development in pubertal females after ovariectomy and stimulation with estradiol/progesterone. In this paradigm, IGFBP-5 KO mammary glands exhibited enhanced alveolar bud formation consistent with enhanced IGF-I action. These results demonstrate that IGFBP-5, although not essential for normal growth, is required for normal mammary gland involution and can regulate mammary gland morphogenesis in response to hormone stimulation.

Time course changes in IGFBP-1 after treadmill exercise and postexercise food intake in rats

Prolonged exercise increases circulating insulin-like growth factor binding protein-1 (IGFBP-1) in humans and animals, but its physiological significance is unknown. This study examined 1) time-course changes in plasma IGFBP-1 and hepatic IGFBP-1 mRNA expression after exercise, 2) changes in IGFBP-1 in relation to plasma glucose, insulin, and IGF-I, and 3) the impact of feeding a postexercise meal on the IGFBP-1 response. Food-deprived male rats were vigorously run on a treadmill and compared with nonexercised controls at 15 min and 1, 4, 8, and 12 h after exercise. Circulating insulin concentrations in exercised rats were lower than in controls at 15 min and 1 h, whereas plasma glucose and IGF-I remained unaffected. Circulating and hepatic expression of IGFBP-1 was markedly increased above that of controls at 15 min, 1 h, and 12 h. In a separate experiment, one-half of the exercised animals received a nutritionally complete meal immediately after the experimental run. The meal elevated plasma insulin and glucose concentrations at 15 min and 1 h. Despite this change in nutritional status, serum IGFBP-1 concentrations and hepatic IGFBP-1 abundance remained elevated at 15 min and 1 h. These results demonstrate that the IGFBP-1 response to a single bout of treadmill exercise is short in duration and independent of insulin, glucose, and amino acid availability.

Dietary restriction of single essential amino acids reduces plasma insulin-like growth factor-I (IGF-I) but does not affect plasma IGF-binding protein-1 in rats

The effects of dietary restriction of a single essential amino acid (EAA) on insulin-like growth factor-I (IGF-I) and IGF-binding protein (IGFBP)-1 were investigated in rats. Rats were fed experimental diets containing amino acid (AA) mixtures in which the concentrations of all EAA were at levels recommended by the National Research Council (control), in which a single EAA was restricted to 20% of that of the control diets (Leu(-), Lys(-), Met(-) or Thr(-)), or in which the diet was devoid of amino acids (AA(-)). To eliminate the effect of differences in energy intake, rats were fed the mean amount of food as consumed by the AA(-) group on the previous day. Growth was significantly retarded in rats fed diets restricted in just one EAA compared with that of rats fed the control diet, and further growth retardation was observed in rats fed the AA(-) diet. On the other hand, the plasma IGF-I concentrations in the groups with a single EAA restriction or in the AA(-) group were 66% (P: < 0. 05) and 50% (P: < 0.05) of that of the control group, respectively. The effect of any single EAA restriction was not significantly different from that of total AA deprivation. The plasma IGFBP-1 concentration in the control group did not differ from that of rats fed diets with the single EAA restrictions except for methionine restriction, but it was approximately 6-fold greater in the AA(-) group. Differences in plasma IGFBP-1 concentration under these conditions could be explained by differences in hepatic IGFBP-1 mRNA contents. Based on these results, we conclude that restriction of single EAA does not affect IGFBP-1 synthesis in vivo, although the deprivation of a single EAA has been reported to increase IGFBP-1 production in hepatocyte cultures. Our results also indicated that a single EAA restriction decreased IGF-I production but did not affect IGFBP-1 production. The present study suggests that not only plasma IGF-I, but also IGFBP-1, affects the magnitude of growth retardation in vivo.

Induction of hepatic insulin-like growth factor binding protein-1 (IGFBP-1) in rats by dietary n-6 polyunsaturated fatty acids

The insulin-like growth factors (IGFs) are mitogenic polypeptides that have been linked to a variety of normal physiological processes as well as neoplasia. Overexpression of several components of the IGF system is associated with hepatocarcinogenesis in humans and rodents. In rat liver, diets rich in n-6 polyunsaturated fatty acid (PUFA) enhance the development of preneoplastic lesions and tumors. Therefore, the objective of this study was to determine the effect of these dietary fatty acids on the hepatic expression of the various components of the IGF system. The mRNA levels of IGF-1 and the type 1 receptor were not different in livers of rats fed a diet containing 20% corn oil (CO) compared with those fed 5% CO. Analysis of the IGF binding proteins revealed that insulin-like growth factor binding protein-1 (IGFBP-1) levels were altered by the amount and type of dietary fat. A 2.5-fold induction of IGFBP-1 mRNA occurred within 1 week after the animals were fed the 20% corn oil diet compared with those fed 5% CO and was further enhanced to over 6-fold after 1 month. Furthermore, IGFBP-1 protein was only detectable in the livers of animals fed the 20% CO diet. Induction of IGFBP-1 mRNA (4.5-fold) also occurred in rats fed a high-fat diet containing safflower (rich in n-6 PUFAs) compared with those fed a high-fat diet containing menhaden oil (rich in n-3 PUFAs). The induction of IGFBP-1 mRNA was independent of serum insulin levels and the development of insulin resistance. Since IGFBP-1 mRNA is upregulated in regenerating liver, we reasoned that the induction of IGFBP-1 mRNA may be associated with an increase in cell proliferation; however, no difference was observed in the hepatic labeling index of rats fed the 20% CO compared with the 5% CO diet. In summary, these studies show a striking induction by dietary n-6 PUFAs of hepatic IGFBP-1, a protein that has been implicated in liver cancer development.

The expression of insulin-like growth factor binding proteins in human hepatocellular carcinoma

Insulin-like growth factors (IGF), IGF receptors and IGF binding proteins (IGFBPs) play an important role in cell growth and differentiation. The liver is the major source of IGF-1 and at least two IGFBPs (IGFBP-1 and IGFBP-3). IGFBPs most often serve to attenuate the effects of IGF at the receptor level and thereby limit IGF-induced cell growth and differentiation. Although changes in IGFBP expression have been described during controlled liver growth such as hepatic regeneration following partial hepatectomy, there is limited knowledge of IGFBPs gene expression in uncontrolled growth or hepatocellular carcinoma. In the present study, we employed Northern blotting techniques to document the expression of IGFBP-1, 3 and 4 in normal human livers, cirrhotic and hepatocellular carcinoma tissues. The results revealed no differences in IGFBP-1, 3 and 4 mRNA levels between normal and cirrhotic tissues. However, the expression of all three IGFBPs mRNA were significantly down regulated in hepatocellular carcinoma tissues. These findings are in keeping with IGFBPs playing an important inhibitory role in the development and/or growth of hepatocellular carcinoma in humans.

Cell-specific localization of insulin-like growth factor binding protein mRNAs in rat liver

The liver is a major source of circulating insulin-like growth factor I (IGF-I), and it also synthesizes several classes of IGF binding proteins (IGFBPs). Synthesis of IGF-I and IGFBPs is regulated by hormones, growth factors, and cytokines. They are nutritionally regulated and expressed in developmentally specific patterns. To gain insight into cellular regulatory mechanisms that determine hepatic synthesis of IGF-I and IGFBPs and to identify potential target cells for IGF-I within the liver, we studied the cellular sites of synthesis of IGF-I, IGF receptor, growth hormone (GH) receptor, and IGFBPs in freshly isolated rat hepatocytes, endothelial cells, and Kupffer cells. We also localized cellular sites of IGFBP synthesis by in situ hybridization histochemistry. Western ligand and immunoblot analyses were used to determine IGFBP secretion by isolated cells. Two IGF-I mRNA subtypes with different 5' ends (class 1 and class 2) were detected in all isolated liver cell preparations. Type 1 IGF receptor mRNA was detected in endothelial cells, indicating that these cells are a local target for IGF actions in liver. GH receptor was expressed in all cell preparations, consistent with GH regulation of IGF-I and IGFBP synthesis in multiple liver cell types. The IGFBPs expressed striking cell-specific expression. IGFBP-1 was synthesized only in hepatocytes, and IGFBP-3 was expressed in Kupffer and endothelial cells. IGFBP-4 was expressed at high levels in hepatocytes and at low levels in Kupffer and endothelial cells. Cell-specific expression of distinct IGFBPs in the liver provides the potential for cell-specific regulation of hepatic and endocrine actions of IGF-I.

Liver-specific and proliferation-induced deoxyribonuclease I hypersensitive sites in the mouse insulin-like growth factor binding protein-1 gene

The insulin-like growth factor binding protein-1 (IGFBP-1) gene is highly expressed in fetal, perinatal, and regenerating liver. Up-regulation is transcriptionally mediated in regenerating liver and occurs in the first few minutes to hours after partial hepatectomy. In transgenic mice a 970-bp region from -776 to +151 of the IGFBP-1 promoter was sufficient for tissue-specific and induced expression of the gene in fetal and hepatectomized livers. However weak and/or poorly regulated expression in some transgenic lines suggested the existence of other regulatory regions. Here, genomic clones containing large regions 5' of the mouse IGFBP-1 gene sequence were isolated, subcloned, and sequenced. Deoxyribonuclease I (DNaseI) hypersensitivity analyses identified clusters of tissue-specific nuclease-sensitive sites in the promoter region, -100 to -300, -2,300, -3,100, and -5,000 along with other weak sites. After partial hepatectomy, enhanced sensitivity and/or novel sites were detected in the -100/-300, -5,000, and -3,100 regions, the promoter region remaining the most hypersensitive. A subset of these sites was present in fetal and perinatal livers. Novel tissue-specific sites that interacted with C/EBP and hepatic nuclear factor 3 (HNF3) transcription factors were identified in the -3,100 region. A hepatectomy-induced DNA binding complex containing the transcription factor USF1 was identified within the -100 to -300 region of the promoter. These results suggested that a complex array of tissue-specific and hepatic proliferation-induced transcription factors combine to regulate both the proximal promoter and more distal regulatory elements of the IGFBP-1 gene.

Changes in the circulating IGF system during short-term fasting and refeeding in rats

There is little information on free insulin-like growth factor I (IGF-I) and its regulatory proteins during fasting and refeeding. Therefore, we examined rats during fasting (0, 1, 2, and 3 days) and refeeding (3, 6, and 12 h and 1, 2, 3, and 7 days) (n = 6-9). Serum was analyzed for insulin, C-peptide, growth hormone (GH), free and total IGF-I, IGF-binding protein (IGFBP)-1 and -3, and the acid-labile subunit (ALS). Additionally, liver mRNA for IGF-I, IGFBP-1, and ALS was determined. Fasting reduced serum levels of GH, free and total IGF-I, IGFBP-3, and ALS, whereas IGFBP-1 was increased (P < 0.0001). Refeeding normalized IGFBP-1 at 3 h and GH at 12 h. Free IGF-I changed in parallel with total IGF-I, ALS, and IGFBP-3, being normalized at 48 h of refeeding. IGFBP-1 (peptide and mRNA) correlated inversely with insulin and C-peptide (P < 0.001). The correlation between peptide and mRNA was relatively strong for IGFBP-1 (r(2) = 0.36; P < 0.0001), moderate for IGF-I (r(2) = 0.18; P < 0.0005), and insignificant for ALS. In conclusion, insulin appears to regulate IGFBP-1 in fasted and refed rats. However, the normal inverse relationship between free IGF-I and IGFBP-1 was absent, and free IGF-I changed in parallel with total IGF-I and thus ALS and IGFBP-3. Finally, the regulation of the hepatic synthesis of IGF-I, IGFBP-1, and ALS seems to differ substantially.

Tissue-specific expression of messenger ribonucleic acids for insulin-like growth factors and insulin-like growth factor-binding proteins during perinatal development of the rat uterus

Insulin-like growth factor (IGF)-I and IGF-II play a number of important roles in growth and differentiation, and IGF-binding proteins (IGFBPs) modulate IGF biological activity. IGF-I has been shown previously to be essential for normal uterine development. Therefore, we used in situ hybridization assays to characterize the unique tissue- and developmental stage-specific pattern of expression for each IGF and IGFBP gene in the rat uterus during perinatal development (gestational day [GD]-20 to postnatal day [PND]-24). IGF-I and IGFBP-1 mRNAs were expressed in all uterine tissues throughout this period. IGFBP-3 mRNA was not detectable at GD-20 but became detectable beginning at PND-5, and the signal intensity appeared to increase during stromal and muscle development. IGFBP-4 mRNA was abundant throughout perinatal development in the myometrium and in the stroma, particularly near the luminal epithelium. IGFBP-5 mRNA was abundantly expressed in myometrium throughout perinatal development. IGFBP-6 mRNA was detected throughout perinatal development in both the stroma and myometrium in a diffuse expression pattern. IGF-II and IGFBP-2 mRNAs were not detected in perinatal uteri. Our results suggest that coordinated temporal and spatial expression of IGF-I and its binding proteins (IGFBP-1,-3,-4,-5, and -6) could play important roles in perinatal rodent uterine development.

Growth hormone promotes somatic and skeletal muscle growth recovery in rats following chronic protein-energy malnutrition

The efficacy of recombinant human growth hormone (GH) and/or a diet enriched in protein and energy to improve growth recovery following prolonged malnutrition was examined in male rats food-restricted from birth until 120 d of age. At d 121, restricted rats were randomly assigned to recovery groups receiving either a control or enriched diet with or without daily subcutaneous injections of GH. Rats were killed after 16 or 47 d of recovery. At d 16, GH treatment stimulated liver, heart, plantaris, soleus, carcass and body weight gain and inhibited fat gain when compared to recovery controls. Rats receiving GH also exhibited the highest serum insulin-like growth factor-I (IGF-I) concentrations and total muscle protein. At d 47, GH effects on body and muscle recovery were minimal, and differences among recovery groups in serum IGF-I concentration and total muscle protein were no longer present. Consumption of an enriched diet increased fat pad and liver mass, but did not promote muscle recovery. There were no differences among treatment groups in skeletal muscle IGF-I mRNA levels at d 16 or 47. In summary, GH had positive effects on somatic and skeletal muscle growth early in the recovery process, possibly via endocrine IGF-I-stimulated protein accretion. In contrast, the enriched diet promoted fat deposition with no impact on skeletal muscle growth recovery.

Hepatic regeneration in insulin-like growth factor binding protein-1 transgenic mice

BACKGROUND/AIMS

Partial hepatectomy results in activation of genes in the residual liver tissue which serve to restore glucose homeostasis and regenerate liver mass. Expression of insulin-like growth factor binding protein-1 (IGFBP-1) is up-regulated following partial hepatectomy and IGFBP-1 can modulate both the metabolic and mitogenic effects of insulin-like growth factor-1 (IGF-I). The aim of the study was to compare the effects of partial hepatectomy on blood glucose levels and hepatic regeneration in wild-type and transgenic mice which constitutively overexpress IGFBP-1.

METHODS

Hepatic IGFBP-1 mRNA, blood glucose concentrations, liver mass and hepatic DNA synthesis were compared in sham-operated and partially hepatectomized transgenic and wild-type mice.

RESULTS

Hepatic IGFBP-1 mRNA was higher in sham-operated transgenic than wild-type mice, but in both groups of mice, partial hepatectomy was associated with a significant rise in IGFBP-1 mRNA. The absolute decline in blood glucose levels following partial hepatectomy was greater in transgenic mice. Basal DNA synthesis and the response to IGF-I in isolated hepatocytes from both groups of mice were similar, and DNA synthesis in the regenerating liver in vivo was not significantly different in transgenic as compared to wild-type mice: 449.3 +/- 63.9 vs. 321.6 +/- 52.3 cpm/microgram DNA. Hepatic regeneration as measured by liver weight after hepatectomy was not different between transgenic and wild-type mice.

CONCLUSIONS

Constitutive overexpression of IGFBP-1 does not enhance hepatic regeneration and does not prevent the decline in blood glucose following partial hepatectomy.

Physiological concentration of amino acids regulates insulin-like-growth-factor-binding protein 1 expression

Protein undernutrition is characterized by growth failure in young growing animals. Current evidence suggests that biosynthesis of insulin-like growth factor (IGF)-I and IGF-binding protein 1 (IGFBP-1) are key control points for nutritional regulation of growth. Here we examined the role of amino acid limitation in regulating the IGFBP-1 expression in the hepatic cell line. Our data show that leucine limitation strongly induces IGFBP-1 without affecting IGF-I and IGF-II expression in human HepG2 cells and in isolated rat hepatocytes. Depletion of arginine, cystine and all essential amino acids leads to induction of IGFBP-1 mRNA and protein expression in a dose-dependent manner. IGFBP-1 expression is significantly induced by leucine concentration in the range of that observed in the blood of rats fed a low-protein diet or in humans affected by kwashiorkor. Moreover, treatment of HepG2 cells with amino acids at a concentration reproducing the amino acid concentration found in portal blood of rats fed a low-protein diet leads to a significantly higher expression of IGFBP-1. These data represent the first demonstration that an amino acid limitation, as occurs during dietary protein deficiency, induces IGFBP-1 expression in hepatic cells. Therefore, amino acids by themselves can play, in concert with hormones, an important role in the control of gene expression.

Expression of insulin-like growth factor-binding protein messenger RNAs in developing rat oligodendrocytes and astrocytes

Insulin-like growth factors, IGF-I and IGF-II, are potent regulators of oligodendrocyte development. Most of the IGF present in vivo is bound to members of a family of six high-affinity IGF-binding proteins (IGFBPs), which can either potentiate or inhibit IGF action, depending on other conditions. Additionally, serum contains a structurally unrelated protein, acid-labile sub-unit (ALS), which forms a ternary complex with IGF and IGFBP3. In this study, we used reverse-transcriptase polymerase chain reaction (RT-PCR) to examine the expression of mRNAs for IGFBP 1-6 and ALS in purified populations of oligodendroglial cells and astrocytes. We found that astrocytes express all six IGFBPs. A2B5+/O4- oligodendrocyte precursors, O4+/O1- intermediate precursors, and O1+ oligodendrocytes express IGFBP3, 5, and 6, while IGFBP4 is expressed in oligodendrocyte precursors but not at more mature stages. We were unable to detect ALS mRNA in whole brain or in cultured oligodendroglial cells. The presence of differentially expressed IGFBPs in developing oligodendrocytes and astrocytes could significantly affect the biological activity of IGF-I and IGF-II in the central nervous system and the IGF-responsiveness of the IGFBP-expressing cells.

Small intestinal disaccharidase activity and ileal villus height are increased in piglets consuming formula containing recombinant human insulin-like growth factor-I

The effect of orally administered IGF-I on intestinal development was assessed in piglets. Cesarean-derived, colostrum-deprived piglets received formula alone or formula containing 65 nM (500 microg/L) of recombinant human IGF-I. IGF-I intake averaged 200 microg/kg/d. On d 7 and 14 postpartum, piglets were killed, organs were removed and weighed, and tissue and blood samples were collected. The small intestine was divided into 13 segments that were weighed and measured. A sample of each segment was fixed in formalin, and the mucosa was scraped for enzyme analyses. Food intake, body and organ weights, intestinal weight, length, protein, DNA and RNA content did not differ between the treatment groups. Serum IGF-I, IGF-II, and IGF-binding protein profiles and tissue IGF-binding protein mRNA expression were also comparable between the treatment groups. In contrast, intestinal enzymes and villus height were increased by oral IGF-I. Lactase was approximately 2-fold higher (p < or = 0.05) in the jejunum and proximal ileum, and sucrase was approximately 50% higher (p < or = 0.05) in the jejunum of IGF-I-treated animals than in controls. Villus height in the terminal ileum was approximately 50% greater in IGF-I-treated animals than in controls (p = 0.03). In conclusion, orally administered IGF-I at 200 microg/kg did not affect whole body or organ growth or serum IGF-I concentrations; however, intestinal disaccharidase activity and ileal villus growth were responsive to orally administered IGF-I, supporting a potential role for milk-borne IGF-I in neonatal intestinal development.

Hypoglycemic effect of insulin-like growth factor-1 in mice lacking insulin receptors

We have investigated the metabolic actions of recombinant human IGF-1 in mice genetically deficient of insulin receptors (IR-/-). After intraperitoneal administration, IGF-1 caused a prompt and sustained decrease of plasma glucose levels in IR-/- mice. Plasma free fatty acid concentrations were unaffected. Interestingly, the effects of IGF-1 were identical in normal mice (IR+/+) and in IR-/- mice. Despite decreased glucose levels, IR-/- mice treated with IGF-1 died within 2-3 d of birth, like sham-treated IR-/- controls. In skeletal muscle, IGF-1 treatment caused phosphorylation of IGF-1 receptors and increased the levels of the phosphatidylinositol-3-kinase p85 subunit detected in antiphosphotyrosine immunoprecipitates, consistent with the possibility that IGF-1 stimulates glucose uptake in a phosphatidylinositol-3-kinase-dependent manner. IGF-1 receptor phosphorylation and coimmunoprecipitation of phosphatidylinositol3-kinase by antiphosphotyrosine antibodies was also observed in liver, and was associated with a decrease in mRNA levels of the key gluconeogenetic enzyme phosphoenolpyruvate carboxykinase. Thus, the effect of IGF-1 on plasma glucose levels may be accounted for by increased peripheral glucose use and by inhibition of hepatic gluconeogenesis. These data indicate that IGF-1 can mimic insulin's effects on glucose metabolism by acting through its own receptor. The failure of IGF-1 to rescue the lethal phenotype due to lack of insulin receptors suggests that IGF-1 receptors cannot effectively mediate all the metabolic actions of insulin receptors.

Insulin-like growth factor binding protein gene expression in the pregnant rat uterus and placenta

While the insulin-like growth factor (IGF) system plays a fundamental role in regulating embryonic and placental growth, the specific contributions of the six IGF binding proteins (IGFBPs 1-6) to these processes are not well understood. We here focus on IGFBP expression in the extraembryonic environment, which both supports and constrains embryonic growth, and have used in situ hybridization to determine sites of IGFBP mRNA synthesis in the pregnant rat uterus and placenta. We find that all IGFBPs are expressed in distinct, changing patterns in the uterine endometrium, at the decidual boundary, in the decidual vasculature, and in the myometrium during pregnancy. Within the endometrium, the most prominent change is that expression of IGFBP-1 begins in some, but not all, endometrial glands prior to implantation and then expands to include all secretory epithelia shortly after implantation. During the period of rapid decidual proliferation that follows implantation, IGFBP-3, -4, and -5 transcripts are all detected in a laminar array at the boundary between the decidua and the nondecidualized endometrium. In the decidual vasculature at Day (d) 8.0, both IGFBP-3 and IGFBP-4 mRNAs are detected in dilating blood vessels, with BP-3 most prominent in the antimesometrial plexus and BP-4 primarily at the mesometrial pole. Later (d11.5), all decidual vessels express high levels of IGFBP-3 and lower levels of IGFBP-4 mRNAs. Finally, changes in expression of several IGFBPs also occur within the myometrium during pregnancy. For example, IGFBP-2 is expressed in the inner circular layer shortly after implantation, and expression increases through late gestation. In contrast, IGFBP-5 hybridization occurs over both myometrial layers before implantation, but decreases in intensity and spatial distribution as pregnancy proceeds. Finally, and most strikingly, IGFBP-6 expression, barely detectable in the d7.0 myometrium, gradually increases until it is very strongly transcribed during the placental stages. Taken together, these observations suggest multiple roles for IGFBPs in supporting implantation, regulating the extent of decidualization, modulating local levels of vascular IGFs, and regulating uterine muscular growth.

Phosphatidylinositol 3'-kinase and p70s6k are required for insulin but not bisperoxovanadium 1,10-phenanthroline (bpV(phen)) inhibition of insulin-like growth factor binding protein gene expression. Evidence for MEK-independent activation of mitogen-activated protein kinase by bpV(phen)

The hormonal regulation of insulin-like growth factor binding protein (IGFBP)-1 and -4 mRNA was compared in serum-free primary rat hepatocyte cultures. The combination of dexamethasone and glucagon (Dex/Gluc) strongly increased IGFBP-1 and IGFBP-4 mRNA levels. Insulin suppressed Dex/Gluc-stimulated IGFBP-1 but not IGFBP-4 mRNA levels. In contrast, the peroxovanadium compound, bisperoxovanadium 1,10-phenanthroline (bpV(phen)), completely abrogated Dex/Gluc induction of both IGFBP mRNA species. Wortmannin and rapamycin blocked the inhibitory effect of insulin but not that of bpV(phen) on Dex/Gluc-stimulated IGFBP mRNA. Thus, although phosphatidylinositol 3'-kinase and p70s6k are necessary for insulin-mediated transcriptional inhibition of the IGFBP-1 gene, a signaling pathway, independent of phosphatidyloinositol 3'-kinase and p70s6k, is activated by bpV(phen) and mediates IGFBP-1 as well as IGFBP-4 mRNA inhibition. Mitogen-activated protein (MAP) kinase activity induced by insulin was suppressed to below basal levels in the presence of Dex/Gluc, whereas in response to bpV(phen), MAP kinase activity was high and unaffected by Dex/Gluc, consistent with a role of MAP kinases in bpV(phen)-mediated inhibition of IGFBP mRNA. The specific MAP kinase kinase (MEK) inhibitor, PD98059, inhibited insulin but not bpV(phen)-stimulated MAP kinase activity, suggesting that MAP kinases can be activated in a MEK-independent fashion. Peroxovanadium compounds are strong inhibitors of tyrosine phosphatases, which may inhibit specific tyrosine/threonine phosphatases involved in the negative regulation of MAP kinases.

Interaction of insulin, glucocorticoids, and protein kinase C in the regulation of insulin-like growth factor-binding protein-1 production by H4IIE rat hepatoma cells

A sensitive RIA was used to examine regulation of IGFBP-1 in H4IIE rat hepatoma cells. IGFBP-1 was stimulated up to tenfold by dexamethasone and corticosterone, and this stimulation was abolished by RU486. The effect of dexamethasone increased with time in culture. Phorbol 12-myristate 13-acetate (PMA) stimulated IGFBP-1 up to fourfold with a maximal effect in short-term culture. Dexamethasone and PMA were additive in stimulating IGFBP-1. Under basal conditions IGFBP-1 production was linearly related to cell density: however, stimulation by dexamethasone was greatest in confluent cells, and PMA had a greater effect in sparse cultures. Insulin inhibited IGFBP-1 up to 80%, and this effect diminished with time in culture but was unaffected by cell density. Dexamethasone was stimulatory in the presence of a maximal inhibitory concentration of insulin, and insulin was inhibitory in the presence of maximal dexamethasone from 3-48 h in culture, regardless of cell density. PMA abolished the inhibitory action of insulin on IGFBP-1 secretion and mRNA expression during incubation periods of less than 4 h and not during longer incubations. PMA did not influence the stability of IGFBP-1 mRNA. We conclude that, in rat H4IIE cells, dexamethasone and PMA stimulate IGFBP-1 by independent mechanisms and speculate that when protein kinase C is activated the inhibitory action of insulin is blocked.

Effect of exogenous gamma-aminobutyric acid (GABA) on hepatic insulin-like growth factor I (IGF-I) and IGF-I binding protein (IGFBP-I) mRNA abundance following partial hepatectomy in rats

Insulin-like growth factor I (IGF-I) and IGF-I binding protein (IGFBP-I) are highly expressed in the liver and may play an important role in enhancing hepatic regeneration following partial hepatectomy in rats. Since hepatic levels of these growth factors are influenced by pituitary growth hormone release, which in turn is regulated by systemic serum gamma-aminobutyric acid (GABA) concentrations, we chose to examine the effects of elevated serum GABA concentrations on hepatic IGF-I and IGFBP-I mRNA abundance following partial hepatectomy in the rat. The results of our experiments revealed that at serum GABA concentrations similar to those associated with liver failure, peak hepatic IGF-I and IGFBP-I mRNA levels were significantly lower in GABA-treated rats compared with saline-treated controls (p < 0.05 and 0.01, respectively). To exclude a direct effect of GABA on hepatocyte IGF-I and IGFBP-I mRNA expression, suspensions of isolated hepatocytes were incubated in the presence and absence of exogenous GABA. In these experiments GABA treatment did not lower either IGF-I or IGFBP-I mRNA or protein levels. These findings suggest that increased concentrations of GABA in the systemic circulation could contribute to the impairment in hepatic regenerative activity that occurs in animals and humans with advanced liver failure.

Tissue specific and cyclic expression of insulin-like growth factor binding proteins-1,-2,-3,-4,-5,-6 in the rat oviduct

Although much is known about the expression insulin-like growth factors (IGF) and their receptors in the murine oviduct, significantly less is known about the expression of IGF binding proteins (IGFBPs). To fill this gap in our knowledge, we identified and characterized the tissue specific expression of IGFBP-1 to-6 in rat oviducts over the estrous cycle byin situ hybridization and immunocytochemistry. Tissues were analysed on proestrus (P1000 h, P2000 h), estrus (E0200, E1000 h), and diestrus I and II (DI 1100 h, DII 1100 h). IGFBP-1 was undetectable in the oviduct over the cycle. IGFBP-2 was selectively expressed in the luminal epithelium. The mRNA levels were high between P2000 h and E1000 h but low or undetectable thereafter. Immunoreactive IGFBP-2 was strong to very strong in these cells over most of the cycle. IGFBP-3 mRNA was undetectable in the oviduct; however, strong hybridization and immunoreactive signals were present in the mesosalpinx and mesotubarium, particularly at DI and DII. IGFBP-4 mRNA was not detected in the oviduct. In contrast, immunoreactive IGFBP-4 was observed in the luminal epithelium and the intensity was very strong after ovulation (E1000 h, DI and DII). IGFBP-5 and-6 mRNAs were selectively expressed in circular smooth muscle cells. Hybridization signals were evident over the cycle, but were greatest at estrus. By comparison, IGFBP-5 and-6 proteins were essentially undetectable in these cells except at DII 1100 h when immunostaining was moderate to high. Luminal epithelial cells were weakly positive for IGFBP-5 and-6. However, intense immunostaining was associated with the ciliated border and the luminal fluid juxtaposed to these cells during the cycle. The oocyte-cumulus complexes were immunostained intensely for IGFBP-2,-4,-5 and-6, but their mRNAs were undetectable. The signals were strongest in degenerating cumulus cells suggesting a potential role for these IGFBPs in cumulus apoptosis. These results demonstrate that the estrous cycle is accompanied by major changes in the pattern of expression of IGFBP-2,-4,-5 and-6 in the rat oviduct. We therefore conclude that the regulated production of these particular IGFBPs may be functionally important in modulating IGF activities in the oviduct, oocyte cumulus complexes, and perhaps the preimplantation embryo as well.

Growth hormone and nutrition interact to regulate expressions of kidney IGF-I and IGFBP mRNAs

The effects of growth hormone (GH) and fasting on renal insulin-like growth factor-I (IGF-I) and IGF-binding proteins (IGFBP)-1, -2, -3, -4, -5 were examined in spontaneous dwarf rats (SDR) which have a complete and specific lack of GH among pituitary hormones. Renal expression of the mRNA which encodes IGF-I was reduced in these rats, and IGFBP-1 and IGFBP-4 were found to be elevated. Administration of GH restored expression of IGF-I and IGFBP-1 mRNA, suggesting that GH is, among other pituitary hormones, more specifically associated with renal expression of these genes. The elevation in the IGFBP-4 mRNA level, however, was not attenuated by GH administration, indicating that this hormone may not be directly related to the regulation of expression of this gene. Fasting for 48 hours resulted in a reduction of IGF-I mRNA and an increase in IGFBP-1 mRNA in SDRs as well as in normal rats, suggesting that a cause other than a reduced serum GH is responsible for these fasting-induced changes. Fasting resulted in little change in levels of other IGFBP-2, -3, -4, -5 mRNAs. When these results were compared with those obtained using liver, IGFBP mRNA expression was shown to be regulated differently in different tissues. Based on our finding that IGFBP-1 modulates the mitogenic action of IGF-I, a full understanding of nutrition-related growth processes in the kidney must take this relationship into consideration as well as that which exists between GH and IGF-I.

Insulin-like growth factor-1 (IGF-1) stimulates the IGF binding protein system in rat theca interstitial cells

There has been considerable interest in rat ovarian insulin-like growth factor binding proteins IGFBPs because they are potent inhibitors of FSH action.In situ, IGFBP-2 and -4 and IGFBP-3 mRNAs are expressed in rat theca interstitial (TIC) and theca lutein cells respectively. Although much is known about IGFBPs in rat TIC at the mRNA level, the synthesis and regulation of IGFBP proteins remain poorly understood. The purpose of this study was to identify the species of IGFBPs produced by TIC and to determine the effects of LH and IGF-1 on their expression. This was accomplished by culturing rat TIC for 2 days in serum-free medium with graded doses of LH and/or IGF-I, and measuring IGFBP mRNAs in the cells and IGFBP proteins in the conditioned media by RT-PCR and Western immunoblotting respectively. The RT PCR analysis identified strong bands for IGFBP-2 and -4 mRNAs in TIC. In some treatments, the mRNAs for IGFBP-3 and -6 were also identified, but transcripts for IGFBP-1 and -5 were undetectable. Two species of IGFBPs were detected in the conditioned media of control (untreated) TIC, the 31 kDa IGFBP-2 and the 24 kDa (non-glycosylated) and 28 kDa (glycosylated) forms of IGFBP-4. There was no detectable IGFBP-5 and barely detectable amounts of IGFBP-3 and -6 in the conditioned media. Treatment with LH (0.2-20 μU/ml) caused no significant changes in the levels of the 31 kDa IGFBP-2 and the 24 kDa and 28 kDa IGFBP-4 bands, and there was no detectable IGFBP protease activity. In contrast, IGF-I (100 ng/ml) stimulated the expression of IGFBP-2, IGFBP-4 and a 17.5 kDa IGFBP-4 fragment. The immunoreactive IGFBP-4 fragment suggests the media contained an IGFBP-4 protease. The IGF-I effects were dose dependent (ED(50)=12.4±3.3 ng/ml). Co-treating TIC with LH (0.2-20 μU/ml) caused no significant change in the activity of IGF-I in stimulating the expression of IGFBP-2, IGFBP-4 and IGFBP-4 protease. We have demonstrated that IGF-I acts directly on rat TIC to stimulate the expression of the intrinsic IGFBP system. LH, either alone or together with IGF-I, did not significantly change the expression of TIC IGFBP proteins. Therefore, we hypothesize that IGF-I, but not LH, may be a physiologically important regulator of the IGFBP system in rat TIC. Because IGF-I is a potent stimulator of theca function, changes in the expression of this intrinsic IGFBP system could have new implications for ovarian androgen production, both at the physiologic and pathophysiologic levels.

The cerebral expression of plasma protein genes in different species

The cerebrospinal fluid (CSF) contains the same proteins as blood plasma, but with a different pattern of concentrations. Protein concentrations in CSF are much lower than those in blood. CSF proteins are derived from blood or synthesized within the brain. The choroid plexus is an important source of CSF proteins. Transthyretin is the protein most abundantly synthesized and secreted by choroid plexus. It determines the distribution of thyroxine in the cerebral compartment. Synthesis of transthyretin first evolved in the brain, then later it became a plasma protein synthesized in the liver. Other proteins secreted by choroid plexus are serum retinol-binding protein, transferrin, caeruloplasmin, insulin-like growth factors, insulin-like growth factor binding proteins, cystatin C, alpha 1-antichymotrypsin, alpha 2-macroglobulin, prothrombin, beta 2-microglobulin and prostaglandin D synthetase. Species differences in expression of the genes for these proteins are outlined, and their developmental pattern, regulation and roles in the cerebral extracellular compartment are discussed.

Human insulin-like growth factor binding protein-1 (hIGFBP-1) transgenic mice: insights into hIGFBP-1 regulation and actions

Three hemizygous transgenic (Tg) mouse lines were generated with a fusion gene composed of the mouse metallothionein promoter (mMT-1) and a full length human insulin-like growth factor binding protein-1 (hIGFBP-1) cDNA that was truncated in its 3' untranslated (3'UT) region. The transgene was ectopically expressed in the brain of each line and resulted in postnatal brain-growth retardation that was manifested by 2 weeks of age. Despite the expression of the transgene in multiple other tissues and high serum hIGFBP-1 concentrations in two of the three lines, studies designed to detect alterations in somatic growth, in reproduction and in glucose metabolism revealed few other abnormalities. Unexpectedly, however, we found that the regulation of the transgene shared characteristics with that of the native gene, despite the fact that it lacked the endogenous gene's 5' regulatory region, as well as most of its 3' UT region. Our studies suggest that factors controlling mRNA stability are important to regulation of both the native and transgene, and that an AU-rich element 17 base pairs (bp) from the end of coding sequence is responsible for the instability of the transgene and in part for instability of the endogenous gene.

cDNA cloning and mRNA expression of the six mouse insulin-like growth factor binding proteins

The insulin-like growth factor binding proteins (IGFBPs) comprise a family of six distinct proteins which modulate insulin-like growth factor action. We have isolated cDNAs encoding the six mouse IGFBPs (mIGFBPs). In addition, we studied the mRNA expression of the six mIGFBPs during development and in various adult tissues. Our results show that each of the six mIGFBPs is highly homologous to their human and rat counterparts, whereas only the N and C terminal ends are conserved between the six mIGFBPs. Northern blotting revealed that mIGFBP-2, -3, -4 and -5 genes are already expressed at gestational day 11.5, suggesting a role for these mIGFBPs in embryonal development. In liver, a peak of mIGFBP-1 mRNA expression was found around birth, suggesting a function for mIGFBP-1 in the newborn mouse. Finally, tissue-specific expression of the six mouse IGFBP genes was observed in adult tissues suggesting different roles or modes of actions in adult life.

Complex pattern of insulin-like growth factor binding protein expression in primary rat osteoblast enriched cultures: regulation by prostaglandin E2, growth hormone, and the insulin-like growth factors

Primary osteoblast-enriched (Ob) cultures from fetal rat bone synthesize insulin-like growth factor (IGF) I and IGF-II, which each enhance Ob function. While a number of agents modulate IGF-I production, IGF-II is constitutively expressed in this culture model. Independent of their expression, however, the activity of the IGFs can be modified by a small group of proteins termed IGF binding proteins (IGFBPs), but little is known about the regulation of individual IGFBPs that are synthesized by Ob cells. Northern blot analysis revealed that serum-deprived primary rat Ob cells express transcripts encoding IGFBP-2, IGFBP-3, IGFBP-4, IGFBP-5, and IGFBP-6, but undetectable levels of IGFBP-1 transcripts. Western ligand blots of Ob culture medium probed with 125I-IGF-I or 125I-IGF-II showed predominant IGFBPs migrating at 30/32 kDa, with minor bands at 24 and 38-47 kDa. Western antibody analysis identified IGFBP-2 and IGFBP-5 within the 30/32 kDa complex, while gel mobility shift on SDS-PAGE following deglycosylation determined that IGFBP-3 comprised the 38-47 kDa complex. By Northern analysis, 6 h treatment with prostaglandin E2 (PGE2), growth hormone (hGH), IGF-I, or IGF-II revealed a complex pattern of regulatory effects on steady-state IGFBP transcript expression. PGE2 increased the transcript levels of IGFBP-3, IGFBP-4, and IGFBP-5, (approximately 22-, approximately 2- and approximately 4-fold respectively), but had no effect on IGFBP-2 or IGFBP-6 transcripts. hGH enhanced IGFBP-3 and IGFBP-5 transcripts (each approximately twofold). IGF-I and IGF-II had no effect on IGFBP-2 steady-state transcript levels but enhanced the level of IGFBP-5 transcripts (approximately fourfold). By Western ligand blot analysis, 24 h treatment with PGE2 elevated the 24 and 38-47 kDa IGFBPs and to a lesser extent the 30/32 kDa complex, hGH elevated the 38-47 kDa IGFBPs, and IGF-I and IGF-II each increased the 30/32 kDa IGFBP complex. Therefore, a comparison of results obtained from Northern, Western ligand, and Western antibody studies indicates that multiple IGFBPs are expressed by primary rat Ob cultures. While IGFBP-2 and IGFBP-6 synthesis in Ob cultures is relatively unaffected by short-term treatment with PGE2, hGH, or the IGFs, these agents modify IGFBP-3, IGFBP-4, and IGFBP-5 expression with individual patterns of effects. In addition, some changes in IGFBP polypeptide levels that are independent of alterations in transcript expression may result from the formation of complexes between IGFs and certain IGFBPs, which could serve to store IGFs for future utilization in the formation phase of bone remodeling.

Differential effects of thermal injury on circulating insulin-like growth factor binding proteins in burn patients

The results of this report provide evidence that insulin-like growth factor-1 binding proteins (IGFBPs) in human sera are differentially regulated as a result of severe burn injury. Using the ligand binding technique, 125I-IGF-1 visualizes 5 different protein bands corresponding to those previously reported for IGFBP-1 to 4 with apparent sizes of 23-42 kd in serum samples prepared from severely burned patients and healthy individuals. The level of IGFBP-3 was significantly decreased within 3-5 days of injury and remained depressed for up to 20 days post injury. The average level of this binding protein reached its lowest value within 3-5 days of the injury (3.8 +/- 1.48% relative to day 0-1 value, n = 4, p < 0.01). Serum samples from 3 of 4 patients showed no recovery within 20 days post injury and the level of IGFBP-3 remained significantly depressed (p < 0.01). In contrast, the levels of IGFBP-2 and IGFBP-4 increased 2 and 3 fold in the same serum samples within 3-5 days of the burn injury, respectively. This increase returns to normal (day 0-1 value) within 7-10 days for IGFBP-2, but the level of IGFBP-4 remained elevated 4 fold relative to the day 0-1 (p < 0.01). However, the abundance of IGFBP-1 in these serum samples was not significantly altered by the burn injury. By controlling for protein loading, these apparent alterations of IGFBPs in the sera of burned patients were not due to hemodilution. Similarly, significant reductions in IGFBP-3 were not likely due to IGFBP-3 specific protease activity in the sera of burn patients since incubation of sera from burn patients and normal individuals at 37 degrees C did not alter the pattern of IGFBPs in sera obtained from normal individuals. Of interest, the level of IGF-1 protein in these samples was also markedly reduced following severe burn injury similar to IGFBP-3. The results of this study suggest that a marked reduction of serum IGF-1 seen in burn patients is associated with a significant reduction of IGFBP-3, a major IGF-1 binding protein in human serum.

Dideoxy sequencing and structural analysis of the rat insulin-like growth factor binding protein-1 gene

Insulin-like growth factor binding protein-1 (IGFBP-1) is an important modulator of IGF bioavailability. To facilitate studies of IGFBP-1 regulation and function in rodent models, we cloned the rat IGFBP-1 gene and analyzed its structure by dideoxy sequencing. The rat IGFBP-1 gene is relatively small (approximately 5 kb) and contains 4 exons and 3 introns, similar to the human IGFBP-1 gene.

Expression and insulin-like growth factor-dependent proteolysis of insulin-like growth factor-binding protein-4 are regulated by cell confluence in vascular smooth muscle cells

Insulin-like growth factor (IGF)-I is markedly induced after balloon injury in the rat aorta, where it may serve to mediate vascular repair. Because the bioavailability of IGF-I is modulated by IGF-binding proteins (IGFBPs), we examined the regulation of IGFBPs by IGFs in primary cultures of rat aortic smooth muscle cells (SMCs). Serum-deprived SMC-conditioned medium contains IGFBPs of 38 to 45 kD (only in confluent cultures), 30 kD (possibly IGFBP-2), 28 kD, and 24 kD (IGFBP-4), the latter being the most abundant. IGF-I and IGF-II but not insulin evoked a marked decrease of IGFBP-4 as early as 4 hours after treatment. IGFBP-4 mRNA abundance, however, was entirely unaffected by IGF-I for up to 48 hours. IGF-I analogues with high affinity for the IGF-I receptor and weak affinity for IGFBP paradoxically evoked a small increase in IGFBP-4, probably through a general increase in protein synthesis. IGF-I only minimally decreased IGFBP-4 content in medium of sparse cultures, whereas it completely abolished IGFBP-4 content in conditioned medium of superconfluent SMCs. IGF-I also evoked a concentration-dependent increase in the abundance of IGFBP-3 in confluent, but not sparse, SMCs without affecting IGFBP-3 mRNA. Addition of IGF-I to cell-free medium conditioned by confluent, but not by sparsely cultured, SMCs led to rapid degradation of IGFBP-4. Interestingly, IGFBP-4 mRNA was markedly induced in confluent relative to sparsely grown SMCs in an IGF-I independent fashion. Thus, both biosynthesis and IGF-dependent proteolysis of IGFBP-4 are increased in confluent SMCs. Proteolysis was maximal at 37 degrees C and was abrogated by EDTA and by benzamidine. Phenylmethylsulfonyl fluoride and the plasmin inhibitor bdellin had minor inhibitory activity, whereas aprotinin, angiotensin-converting enzyme inhibitors, and N-ethylmaleimide were without effect. The protease does not affect the structure of IGF-I as determined by reverse-phase high-performance liquid chromatography and size-exclusion chromatography of 125I-IGF-I incubated for up to 24 hours with SMC-conditioned medium containing IGFBP-4. In summary, SMCs elaborate a cation-dependent protease in a confluence-dependent fashion, which degrades bound IGFBP-4 and likely releases free structurally intact IGF-I, presumably to interact with the cell surface receptor and/or other IGFBPs.

Circulating insulin-like growth factor binding proteins (IGFBPs) 1 and 2 induced in vitamin C-deficient or fasted guinea pigs inhibit IGF-I action in cultured cells

Collagen gene expression and proteoglycan synthesis are decreased in vitamin C-deficient guinea pigs losing weight and in fasted guinea pigs receiving ascorbate. Sera from such guinea pigs contain an insulin-like growth factor (IGF)-I-reversible inhibitor of collagen, proteoglycan and DNA synthesis and elevated levels of 29 and 35-kDa IGF binding proteins (IGFBPs). We now have identified the induced proteins as IGFBPs 1 and 2 and investigated their role as inhibitors. Guinea pig sera were treated with antibodies to IGFBPs 1 and 2 and antibody-IGFBP complexes were removed by passage through a Protein A-Sepharose column. Inhibitor content of fasted and scorbutic sera, and Protein A pass-through fractions derived from them, was assessed by their level of stimulation of DNA and collagen synthesis in 3T3 cells, compared to analogously treated normal guinea pig serum. Removal of IGFBP-1 from scorbutic serum reversed inhibition of collagen and DNA synthesis by more than half but removal of IGFBP-2 was less effective. Removal of both IGFBPs reversed inhibition almost completely. Similar results were obtained with fasted guinea pig serum. Conversely, purified rat IGFBPs 1 and 2 inhibited DNA and collagen synthesis in cells cultured in normal guinea pig serum or IGF-I-stimulated DNA synthesis, with IGFBP-1 being more potent. Thus, IGFBP-1 and, to a lesser extent IGFBP-2, cause inhibition of IGF-I action by sera from fasted and scorbutic guinea pigs and may inhibit collagen gene expression in vivo.

Tissue-specific expression of the insulin-like growth factor binding protein (IGFBP) mRNAs in mouse and rat development

The insulin-like growth factor binding proteins (IGFBPs) are polypeptides which are thought to modulate the bioactivity of IGF-I and IGF-II, and may also have activities independent of the IGFs. The expression patterns of IGFBPs-1, -3, -4, and -6 in midgestational rodents were analyzed using in situ hybridization to begin to characterize the role of these IGFBPs during development. All IGFBPs are expressed at least as early as rat embryonic day 14 (e14), and each has a unique pattern of expression. IGFBP-1 mRNA is expressed by the liver throughout mid and late gestation. IGFBP-3 mRNA is expressed at high levels in the urogenital tract, several muscle groups, and the nasal epithelia. IGFBP-3 transcripts are also expressed at lower levels by many non-neural tissue types, including the liver, stomach, and heart. IGFBP-4 is abundantly expressed by many tissues in the developing embryo, with the notable exceptions of the spinal cord, specific cartilage groups, and the thymic cortex. IGFBP-6 is expressed in the liver by e14, and also by a previously unrecognized cell population surrounding developing cartilage. Taken together these observations suggest distinct roles in development for each of the IGFBPs.

Expression and regulation of insulin-like growth factor binding protein-1 in the rat uterus throughout estrous cycle

In previous studies, we have demonstrated that IGF-1 and IGF-1 receptor are expressed in rat uterus and that the expression is up-regulated by estrogen. The present study examines the expression and regulation of IGFBP-1 and IGFBP-3 in rat uterus throughout the estrous cycle. The stage of the estrous cycle in 16 mature female rats was determined by microscopic examination of daily prepared vaginal smears. Rat uteri were then used for RNA extraction. The results of the Northern blot analysis demonstrate that uterine cells express both IGFBP-1 and IGFBP-3 mRNA throughout the estrous cycle. When autoradiograms were quantitated by a densitometry, a significant reduction in expression of IGFBP-1 mRNA was found in uteri at stages of proestrous and estrous relative to that in diestrus. Although the level of IGFBP-3 mRNA varied in uteri throughout estrous cycle but this variation was not statistically significant. The lowest expression of IGFBP-1 (8.5% relative to diestrus, p < 0.05, n = 4) and IGFBP-3 (71% relative to diestrus) was found in the uteri prepared from rats at the stage of proestrus, while the highest expression of IGFBP-1 and IGFBP-3 was observed in the uteri obtained from rats at the stage of diestrus and metestrus, respectively. Using anti-rabbit IGFBP-1 antibody raised against an oligo-synthetic IGFBP-1 peptide, immunohistochemical staining demonstrates the presence of IGFBP-1 in the luminal and stromal glandular epithelial cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential expression of insulin-like growth factor binding proteins (IGFBP) 4 and 5 mRNA in the rat brain after transient hypoxic-ischemic injury

Recent studies suggest a role for the insulin-like growth factor (IGF) system in the repair of damaged tissue following hypoxic-ischemic injury in the infant rat brain. We have used a unilateral model of hypoxic-ischemic injury to assess the possible involvement of two IGF binding proteins (IGFBPs), IGFBP-4 and IGFBP-5, in the post-asphyxial response. Ligation of the right carotid artery of 21-day-old rats was followed by either 15 min or 60 min exposure to 8% oxygen to produce moderate and severe damage respectively. Using in situ hybridization, the distribution of IGFBP-4 and IGFBP-5 mRNA was determined in brains collected over 10 days following the insult. In the control brains (no damage), both IGFBPs were expressed in distinct regions. IGFBP-4 mRNA was detected in limited areas of the hippocampus and in several cortical layers, while IGFBP-5 mRNA was found primarily in the thalamus. In response to hypoxic-ischemic injury, IGFBP-4 mRNA expression was reduced in regions of neuronal loss, suggesting a neuronal origin for IGFBP-4. The expression of IGFBP-5 mRNA was not altered by the 15 min insult, but was heavily induced from 3 days following the 60 min insult, particularly in the subependymal layer and adjacent white matter on the ligated hemisphere. This suggests that IGFBP-5 may be involved in recovery from severe hypoxic-ischemic injury and may be important in the regeneration of oligodendrocytes.

HNF1 activates transcription of the human gene for insulin-like growth factor binding protein-1

Insulin-like growth factor binding protein-1 (IGFBP-1) is expressed primarily in the liver, kidney, and uterus. Basal IGFBP-1 promoter activity in human HEP G2 hepatoma cells is dependent upon a proximal promoter element that binds hepatic nuclear factor 1 (HNF1), a protein that is likely to be an important factor regulating the expression of many genes in liver and kidney. To test whether HNF1 activates IGFBP-1 transcription, HEP G2 cells and HeLa cells were cotransfected transiently with HNF1 expression vectors and with IGFBP-1 promoter/chloramphenicol acetyltransferase reporter gene constructs. HNF1 increased IGFBP-1 promoter activity in both HEP G2 and HeLa cells. Gel mobility-shift assays and additional transfections in HeLa cells showed that expressed full-length and carboxy-terminal truncated forms of HNF1 could each bind the HNF1 cis element of the IGFBP-1 promoter; however, significant trans-activation only occurred in the presence of the full-length HNF1 protein, similar to past experience with these two HNF1 forms and the albumin promoter. Further studies showed that IGFBP-1 promoter constructs containing mutations with high or low affinity for HNF1 responded to HNF1 expression with increased or decreased activity, respectively, relative to the native promoter. These studies suggest that HNF1 and/or related proteins play a role in hepatic, and perhaps also renal, expression of IGFBP-1.

Interactive effects of nutrients and hormones on the expression of insulin-like growth factor binding protein-1 (IGFBP-1) mRNA and peptide, and IGF I release from isolated adult rat hepatocytes

Isolated adult rat hepatocytes were used to investigate and compare the actions of glucose or amino acids and insulin, glucagon, growth hormone, and dexamethasone on the expression of insulin-like growth factor binding protein (IGFBP) mRNA, or the release of IGFBP and IGF peptides in vitro. Ligand blot analysis of culture medium conditioned for 24 h by monolayers of hepatocytes in the presence of 6.5 mM glucose revealed two species of IGFBPs, an abundant form of 30-32 kDa and a minor species of 22-24 kDa. Western blotting showed that two IGFBPs of 29-30 and 32 kDa were recognized by antiserum against hIGFBP-1, whereas hepatocytes contained a 1.6 kb transcript on Northern blot with a rat IGFBP-1 cDNA. Insulin-like growth factor BP-2 mRNA was not detected in hepatocytes and IGFBP-2 immunoreactive peptide not present in conditioned medium. The release of IGFBP-1, determined by ligand blot, was independent of glucose concentration over the range of 2.7 mM-11.1 mM, but IGFBP-1 mRNA was decreased following incubation with 6.5 mM glucose compared with 2.7 mM glucose. The release of IGFBP-1 by hepatocytes was inhibited by insulin (10 nM-1 microM), as was mRNA abundance. However, these effects of insulin on IGFBP-1 diminished with increasing glucose concentration. Increasing concentrations of total amino acids increased IGFBP-1 release as did dexamethasone (100 pM-100 nM), whereas growth hormone and glucagon were without effect. The release of IGF I was increased by insulin, growth hormone and dexamethasone but was decreased by glucagon and amino acids, whereas changes in glucose concentration had no effect. The results show that isolated adult rat hepatocytes release IGF I and IGFBP-1 under the interactive control of nutrients and hormones involved in metabolic homeostasis.

Effect of protein nutrition on the mRNA content of insulin-like growth factor-binding protein-1 in liver and kidney of rats

Effect of quantity and nutritional quality of dietary proteins on the content of mRNA of insulin-like growth factor-binding protein-1 (IGFBP-1) was studied in rat liver and kidney. IGFBP-1 mRNA content per unit RNA increased in liver and kidney of rats fed on a protein-free diet and in those of fasted rats compared with that in the rats fed on a casein diet. When rats were given a gluten diet for 7 d, IGFBP-1 mRNA content in liver did not change significantly but that in kidney increased considerably compared with that in those organs of the rats fed on the casein diet. Because IGFBP-1 mRNA has been demonstrated both in liver parenchymal and non-parenchymal cells (Takenaka et al. 1991), the effect of the protein-free diet on these two types of cells has been studied. An increase in IGFBP-1 mRNA content under protein deprivation was observed in both liver parenchymal and non-parenchymal cells, suggesting that these two types of cells are regulated in a similar mode as far as IGFBP-1 mRNA content is concerned. The physiological and nutritional significance of the previously stated results on protein anabolism are discussed when considered together with our previous observations on the plasma concentrations of IGF-1 (Takahashi et al. 1990) and IGFBP (Umezawa et al. 1991) and insulin-like growth factor-1 mRNA content in liver (Miura et al. 1991).

Possible autocrine/paracrine actions of insulin-like growth factors during embryonic development: expression and action of IGFs in undifferentiated P19 cells

The insulin-like growth factors I and II (IGF I and II) and their cell surface receptors are expressed in the mammalian embryo and may function as autocrine or paracrine growth factors during early development. P19 embryonic carcinoma cells, derived from a 7.5 day mouse embryo, were used as a model for a functional study of the IGF system in post-implantation embryogenesis. Undifferentiated P19 cells synthesized IGF I and II, the type I and II IGF receptors, and IGF binding proteins (IGF BP2, IGF BP3, and IGF BP4). P19 cells showed an increase in thymidine incorporation of 150% of control with a 4 hour incubation of IGF I (10 ng/ml) or IGF II (100 ng/ml) and an increase in cell viability compared to control cells during 24 hours of serum starvation. In both experiments IGF I was more potent than IGF II. Endogenous concentrations of IGF I and II in conditioned media were low compared to the doses of exogenous IGFs required for biologic effect, but nonetheless contributed significantly to baseline DNA synthesis, as demonstrated by inhibition of IGF actions with specific antibodies. Cell surface associated IGF BPs bound more radiolabeled IGF than IGF receptors, as determined by binding studies and affinity cross-linking. IGF I and IGF II appeared to regulate production of IGF BP2, suggesting that the IGFs may regulate their own actions by altering the abundance of their binding proteins.

The role of insulin-like growth factors and IGF-binding proteins in the physiological and pathological processes of the kidney

Insulin-like growth factors (IGFs) and their binding proteins are implicated in the growth regulation of the kidney during embryogenesis and differentiation. Recent evidence also suggests that IGFs play a role in kidney physiology (glomerular filtration rate, renal plasma flow) and pathology (diabetic renal hypertrophy, nephritis, glomerulosclerosis, kidney tumours, chronic renal failure). This review focuses on the biology of IGFs at the molecular, protein and receptor levels and considers their importance in renal physiology and pathology. The current data demonstrate a central role for the IGFs in the mediation of a wide variety of effects on renal growth, function and malignancy.

Platelet-derived growth factor isoforms decrease insulin-like growth factor I gene expression in rat vascular smooth muscle cells and selectively stimulate the biosynthesis of insulin-like growth factor binding protein 4

Platelet-derived growth factor (PDGF) is believed to be a critical mediator of vascular smooth muscle cell (SMC) proliferation. Because insulin-like growth factor (IGF) I (IGF-I) functions as a progression factor for the mitogenic effects of PDGF, we hypothesized that IGF-I gene expression and the production of IGF binding proteins (IGFBPs) by cultured rat aortic SMCs might be regulated by one or more of the three isoforms of PDGF: PDGF-AA, -BB, and -AB. IGF-I gene expression was highly dependent on cell density: IGF-I mRNA transcripts decreased markedly as a function of cell confluence. IGF-I mRNA content was inhibited to a similar degree by PDGF-AA, -BB, and -AB through a mechanism requiring protein synthesis. The inhibition was readily apparent at 4 hours, reaching approximately 25% of control levels after 24 hours. Radioimmunoassayable IGF-I was only barely detectable in SMC-conditioned serum-free medium and not significantly modulated by PDGF. Western ligand blot revealed that vascular SMCs release 30-kd and 24-kd IGFBP into serum-free conditioned medium. PDGF isoforms did not significantly alter release of the 30-kd IGFBP but evoked a fivefold to sixfold increase in the 24-kd IGFBP. The 24-kd IGFBP was found to comigrate with IGFBP-4, a recently identified binding protein that inhibits IGF action. The 30-kd protein was not merely a glycosylated form of IGFBP-4, because it was not sensitive to N-glycanase digestion. PDGF-AA, -BB, and -AB markedly induced expression of IGFBP-4 mRNA in a time- and concentration-dependent fashion. Vascular SMCs also express IGFBP-2 mRNA, but its abundance was not induced by PDGF. In conclusion, PDGF evokes a complex pattern of regulation of genes in the IGF/IGFBP system. By inhibiting IGF-I production and specifically inducing biosynthesis of the inhibitory binding protein IGFBP-4, PDGF may set in motion mechanisms to limit the final magnitude of the mitogenic response.

Cloning of the rat insulin-like growth factor binding protein-1 gene and analysis of its 5' promoter region

To understand specific mechanisms involved in the regulation of insulin-like growth factor binding protein-1 (IGFBP-1), an important modulator of IGF bioactivity, we cloned the rat IGFBP-1 gene and sequenced a 1.5 kb Sph1-Sph1 fragment containing 1110 bases upstream from the translation start site. Computer analysis reveals the presence of ATA, CACCC, and CCAAT elements, and putative homeodomain, AP-1, insulin and glucocorticoid response elements in the 5' promoter. Primer extension and ribonuclease protection studies reveal a single cap site in RNA from rat hepatoma cells and both control and diabetic rat liver.

Effects of partial hepatectomy on hepatic insulinlike growth factor binding protein-1 expression

Insulinlike growth factor binding proteins modulate the action of the insulinlike growth factors in various bioassays and may regulate the bioavailability of the insulinlike growth factors in vivo. Because the insulinlike growth factors may influence hepatic regeneration, we have examined the effect of partial hepatectomy on serum insulinlike growth factor binding proteins and on the abundance of insulinlike growth factor binding protein-1 messenger RNA in the liver. All rats were fasted before and after partial hepatectomy or sham operation to avoid the confounding effects of difference in food intake. Using a conventional protocol, 70% of the liver was removed, and groups of four or five rats were killed at different intervals after partial hepatectomy. Sham-operated rats served as controls. Pooled sera from each group of rats were analyzed by ligand blotting with 125I-insulinlike growth factor-I. Liver RNA from individual rats was analyzed by slot-blot and Northern-blot hybridization. A small decrease in the 39- to 42-kD insulinlike growth factor binding protein was apparent in sera from both the sham-operated and partial hepatectomized rats. In contrast, a dramatic increase (fivefold) in the 29-kD serum insulinlike growth factor binding protein (insulinlike growth factor binding protein-1) was apparent only in the partial hepatectomized rats. Hepatic insulinlike growth factor binding protein-1 messenger RNA abundance was significantly increased (1.99 +/- 0.18-fold; p less than 0.05) at 1 hr, reached a peak of 2.32 +/- 0.22-fold (p less than 0.01) at 3 hr after partial hepatectomy and returned to basal levels over the subsequent 6 to 12 hr.(ABSTRACT TRUNCATED AT 250 WORDS)

Contiguous localization of the genes encoding human insulin-like growth factor binding proteins 1(IGBP1) and 3(IGBP3) on chromosome 7

In extracellular fluids the insulin-like growth factors (IGFs) are bound to specific binding proteins (IGBPs). The genes for two members of this protein family have been mapped, the IGBP1 gene to human chromosomal region 7p14-p12 and the IGBP2 gene to region 2q33-q34. In this study, somatic cell hybrid analysis indicated that IGBP3 is also located on chromosome 7. Pulsed-field gel electrophoresis was used to demonstrate the close physical linkage between IGBP1 and IGBP3. Overlapping cosmid clones encompassing these genes were isolated, and restriction endonuclease mapping showed that the genes are arranged in a tail-to-tail fashion separated by 20 kb of DNA. Further characterization of the IGBP1 DNA sequence disclosed a duplication of the intron 3-exon 4 junction within the third intron. In addition, we report RFLPs for ApaLI and TaqI in the IGBP1 locus.